The recent trends toward lighter, smaller, and higher-density electronic devices have increased the demands for various printed circuit boards. In particular, the demands for flexible laminates (also referred to as “flexible printed circuit boards (FPC)”) are showing a notable increase.
The flexible laminate includes an insulating film on which a circuit made from a metal foil is formed. The flexible laminate is generally produced by laminating and pressing with heat a metal foil onto a substrate thorough an adhesive material, the substrate being a flexible insulating film composed of an insulating material.
A polyimide film or the like is preferred for the insulating film application.
A thermosetting adhesive, such as an epoxy or acrylic adhesive, is typically used as the adhesive material. (An FPC prepared using the thermosetting adhesive is hereinafter also referred to as a “three-layer FPC”.)
An advantage of the thermosetting adhesive is that bonding at relatively low temperature is possible. However, the requirements for various characteristics, such as heat resistance, flexibility, and electrical reliability, are becoming more and more stringent. It is possible to say that the three-layer FPC using the thermosetting adhesive may not be able to meet these stringent requirements.
To overcome this problem, an FPC prepared by directly laminating a metal layer onto an insulating film (polyimide film) or by using a thermoplastic polyimide in the adhesive layer has been proposed. Hereinafter, this type of FPC is also referred to as “two-layer FPC”. Two-layer FPC have property superior to those of three-layer FPC and the demands therefor are expected to grow in the future.
Examples of methods for making flexible metal-clad laminates for use in two-layer FPC include a casting method in which a polyamic acid, i.e., a precursor of polyimide, is flow-cast onto a metal foil, followed by imidization; a metallizing method in which a metal layer is directly formed on a polyimide film by sputtering, plating, or the like; and a lamination method in which a thermoplastic polyimide is used to bond a metal foil onto a polyimide film.
Among these methods, the lamination method is superior to the others in that the range of the thickness of the metal foils usable in this method is wider than that in the cast method and that the equipment cost is lower than that of the metallizing method. Examples of the equipment for the lamination include a hot roll laminator and a double belt press machine that can continuously conduct lamination while unreeling a material wound into a roll. Of these, the hot-roll laminator is preferable from the standpoint of productivity.
According to a conventional process for preparing a three-layer FPC by the lamination method, a thermosetting resin has been used to form the adhesive layer. Thus, lamination at less than 200° C. has been possible (refer to Japanese Unexamined Patent Application Publication No. 9-199830). In contrast, since the two-layer FPC uses a thermoplastic polyimide in the adhesive layer, a high temperature of at least 200° C. and sometimes near 400° C. is necessary in order to yield thermal bondability. Thus, a flexible metal-clad laminate produced by the lamination suffers from residual strain, which causes dimensional change when wiring is formed by etching or when solder reflow is conducted to mount a component.
For example, in a lamination method, a polyamic acid, which is a precursor of the thermoplastic polyimide, is flow-cast or applied, and continuously heated to perform imidization so as to form a thermoplastic polyimide-containing adhesive layer on a polyimide film, and then a metal foil is bonded thereon. Since heat and pressure are continuously applied not only in the step of imidization but also in the step of bonding the metal layer, the material is frequently placed in a high-temperature environment with a tension applied on the material. The tension is released in the step of etching the metal foil of the flexible laminate and in the step of heating during solder reflow; therefore, the dimensions frequently change before and after these steps.
Ever increasing demands for miniaturization and weight reduction of electronic components have also promoted development of finer wiring to be formed on a substrate. Components mounted on the substrate are also required to achieve miniaturization and higher density. If the dimensional change after such fine wiring is formed is large, the position of the component mounted may deviate from the position originally designed, thereby generating problem such as defective coupling between the components and the substrate.
Thus, various attempts have been made to suppress the dimensional change by controlling the lamination pressure, by controlling the tension applied to the adhesive film (refer to Japanese Unexamined Patent Application Publication Nos. 2002-326308 and 2002-326280).
Although the techniques disclosed in these publications improve dimensional changes, the degree of improvement is not sufficient. A further improvement on dimensional changes is desired.